If you’re going to get a MacBook (usually around $999 at the Apple Store), I’d go ahead and get the extra 2 GBs of RAM (for a total of 4 GBs) for $100 more. It will make a difference in the heat that the MacBook gives off and how much the hard drive and cooling fans have to work. And it will help with speed, too, for all your work. It’s not that much more to spend, and will pay off in terms of usability, and over the lifetime of the MacBook, you will forget about the extra money it cost. I’ve seen MacBooks go from being noisy and hot with their fans running all the time to being quiet machines with an extra 2 GBs of RAM.
If you’re going to get a MacBook Pro, it will already have 4 GBs in it, and the only other option is to go to 8 GBs for $400. 4 GBSs should be enough for most users, unless you want the MacBook to be as fast and stay as cool as possible, and you’re going to be working with photos or digital videos. $400 is pretty steep, considering you can get that same memory from an OEM dealer for around $150 and put it in yourself. But the Apple installed memory is fully guaranteed, too, by Apple.
It’s a bit more difficult to put more memory in a MacBook or MacBook Pro than it used to be. But if you want to install more memory in a new MacBook or Pro to save money, or put some extra in an older model, Apple has instructions at their website. See MacBook: How to remove or install memory and MacBook Pro: How to remove or install memory.
A good memory dealer is called Other World Computing, and they have good deals on memory and free installation videos, too. See MacBook and MacBook Pro Memory Upgrades.